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balancing a full flying stab
Guys,
in continue to this problem: http://www.rcuniverse.com/forum/fb.asp?m=8870937 i consulted my friend and he adviced i need to check the balance (CG) of the full flying elevon stabs, and when hanged by the pivot point, the stab should be about level and even slightly nose down. can you express your thoughts about this matter ? thanks. |
RE: balancing a full flying stab
In balance is best. If not in balance then there is a constant load on the servo to hold it in position and the servo will be buzzing and drawing current. The weight of any imbalance is proportional to the G load so if the servo is having to hold against a weight of X ounces in level flight then it has to hold against a load of 6 times X ounces in a 6G turn/loop, so what may be a couple of ounces of pull to hold it in level flight becomes almost a pound of pull from the servo in a 6G loop, just to hold the weight imbalance. Also any imbalance can come as flutter when the plane is disturbed by gusts and turbulence which is why many full size have mass balances on control surfaces - if you shake the plane up and down a balanced surface stays level giving no flutter or load on the servo, if the surface is not balanced it will flap about causing a load on the servo and flutter if the frequency hits the right rate.
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RE: balancing a full flying stab
Goodmorning Lior! ...cant see you online in MSN..[8D]
i hope this posts will help you about stab balancing : http://www.rcuniverse.com/forum/m_68...%2Cstab/tm.htm stab balancing takes out the force from the waith of the sufrace to the servo..so only airpresure force is left for the servo to control . also less mass to hold in center while moving and on hard landings no force stress servo gears and servo base .. my 2 c |
RE: balancing a full flying stab
Harry, surely in normal level flight at X mph aerodynamic forces would take any inbalace loads out of the equation. It's only when its static and low speeds that there is any load on the servo.
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RE: balancing a full flying stab
ORIGINAL: madmodelman Harry, surely in normal level flight at X mph aerodynamic forces would take any inbalace loads out of the equation. It's only when its static and low speeds that there is any load on the servo. If you put the pivot point behind the AC then the stab becomes unstable and will try to flip backwards, generating huge forces on the servo. If the pivot is ahead of the AC the stab will be stable and yes the airflow will try to centre it thus helping to counter any weight imbalance, but the aerodynamic force will increase with the amount of travel and with airspeed and requires a stronger and stronger servo and linkages to overcome it. H PS the above applies to a stab which has a symmetrical section. If it is non symmetrical then it will generate a rotating force around the AC. For example, the F-4 Phantom and Tornado are inverted sections which will generate a leading edge upwards rotation, the amount of torque being dependent upon airspeed so even if mass balanced they will create a force for the servo even at neutral which gets strogner as you go faster. |
RE: balancing a full flying stab
Understood.
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RE: balancing a full flying stab
this guy knows his stuff.. said it perfect!!!!!!!
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RE: balancing a full flying stab
So how would you guys recommend balancing the stab ? shall i let it pivot freely on the shaft by using a bearing of some sort, and then add lead weight in the root of the stab in the leading or trailing edge until the CG is right on the pivot ? or is there some other way ?
Shall the CG be right on the pivot or slightly in front of it towards the leading edge ? |
RE: balancing a full flying stab
Add weight in the root. Balance it at the pivot, not in front.
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RE: balancing a full flying stab
ORIGINAL: HarryC Add weight in the root. Balance it at the pivot, not in front. |
RE: balancing a full flying stab
1Eye, could you explain how you found that 27% is best? I've read that the neutral point (assuming a symmetric airfoil) is typically at 25% of MAC. I ask because I'm considering a flying stab for a future project. Thanks
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RE: balancing a full flying stab
ORIGINAL: 1Eye I didn't see anybody mention the need to determine MAC (mean aerodynamic chord) yet. I've found the ''sweet spot'' for the pivot (and therefore balance) point is around 27% relative to the MAC. ORIGINAL: HarryC The ideal location of the pivot is at the aerodynamic centre. |
RE: balancing a full flying stab
1 Attachment(s)
Hi
A while back I was sent an .xls file to find the center of the rotating axis of a flying stab (or canard). Still I am flying my F18 with unbalanced stabs and I have over 50 flights now. The buzz in the servos is big when the plane is in the ground and I was planning to find the rotating axis with that file. Although, what is buzzing me is that the rotating axis is not the pressure center during flight and that is what kept me from balancing the stabs. In the end - I just don't know where to balance. I want to balance in the pressure center and not in the c.g. of the stab. The PC, should be backwards of the rotating axis and the actual stab pivot of the F18 flying stab, is too much forward of both PC and rotating axis. Imputs welcomed [b]Rowing width 600 mm Root rib [c] 500 mm Outer rib [a] 300 mm Edges rear differential [d] 100mm Output data t/4 plus 5% 322 mm Spin axis of the root trailing edge rib [e] 276mm Note: d is at angles> 90 ° is positive, at 90 ° is equal to 0, with <90 ° is negative, eg. -100 |
RE: balancing a full flying stab
ORIGINAL: jetnuno Still I am flying my F18 with unbalanced stabs and I have over 50 flights now. I want to balance in the pressure center and not in the c.g. of the stab. The PC, should be backwards of the rotating axis and the actual stab pivot of the F18 flying stab, is too much forward of both PC and rotating axis. I think you mean the Aerodynamic Centre, the Centre of Pressure is a different thing, it is a combination of lift and pitching and it moves about a lot. The AC of a stab or a canard is the same as for a wing, roughly at 25% of the MAC. The exact point of the AC depends on the wing section being used, if you plot the AC position for a lot of sections you find the majority clustered close around 25%, a few out around 24 or 26% and a very few out near 23 and 27%. Thus for model flying purposes it works to use the approximation of 25% A model may replicate the position of the pivot and balance of the full size but that may be totally unsuitable for a model! We have to remember that aircraft like the full size F-18 will be operating in the transonic and supersonic regions where airflows and centres of pressure change enormously compared to our strictly subsonic models. The full size will have some features designed to cope with the dramatic changes that we will not eno****er. It could well be optimised for cruising in the transonic region and therefore be utterly wrong shape for our subsonic flight. Also the full-size is designed by very clever professionals with super computers, ours are designed by amateurs using rule-of-thumb approximations, so the full-size can do things quite differently because they have calculated exactly what is happening while we are guessing. And finally the full size is destabilised and as a result is fly by wire, whereas although our model is the same shape our model is most definitely stable with amuch more forward CG and does not have the luxury of fly-by-wire. So if we make a model of Spitfire which operates in the same subsonic region as us and has the same classic controls, we can make our model using the same rules. But when we model something that is supersonic and destabilised and fly-by-wire, but we still have to use the same Spitfire rules of subsonic, stable and classic control, we are going to see things that look bizarre to us and wonder if we should emulate the pivot point and balance of the full-size. H |
RE: balancing a full flying stab
Hi Nuno,
Could you send me this excel sheet ? I'd like to see if it gives good results. I have balanced the F-18F stab at 25% MAC. However I have determined the MAC empirically. I'd be interested be look at the formula. Thank you. P.S: The balancing I have done is working perfectly on the F-18F BTW. |
RE: balancing a full flying stab
I don't know about all of the math that goes into calculating the balance point but I can tell you from my building and flying experience with planes that incorporate full flying stabs (which include a Tamjets F-18, two 1/8 f-16's, an F-100, and a Scorpion) that most manufacturers do not recommend fully balancing the stabs. Let them slightly fall down at the front of the stab when they are sitting in the fuselage and you will not have any issues. All of mine have been balanced in this manner and all work well. All of mine have the lead added at the leading edge of the stab at the root.
Gary |
RE: balancing a full flying stab
ORIGINAL: Bob R2 1Eye, could you explain how you found that 27% is best? I've read that the neutral point (assuming a symmetric airfoil) is typically at 25% of MAC. I ask because I'm considering a flying stab for a future project. Thanks |
RE: balancing a full flying stab
ORIGINAL: HarryC ORIGINAL: jetnuno Still I am flying my F18 with unbalanced stabs and I have over 50 flights now. I want to balance in the pressure center and not in the c.g. of the stab. The PC, should be backwards of the rotating axis and the actual stab pivot of the F18 flying stab, is too much forward of both PC and rotating axis. I think you mean the Aerodynamic Centre, the Centre of Pressure is a different thing, it is a combination of lift and pitching and it moves about a lot. The AC of a stab or a canard is the same as for a wing, roughly at 25% of the MAC. The exact point of the AC depends on the wing section being used, if you plot the AC position for a lot of sections you find the majority clustered close around 25%, a few out around 24 or 26% and a very few out near 23 and 27%. Thus for model flying purposes it works to use the approximation of 25% A model may replicate the position of the pivot and balance of the full size but that may be totally unsuitable for a model! We have to remember that aircraft like the full size F-18 will be operating in the transonic and supersonic regions where airflows and centres of pressure change enormously compared to our strictly subsonic models. The full size will have some features designed to cope with the dramatic changes that we will not eno****er. It could well be optimised for cruising in the transonic region and therefore be utterly wrong shape for our subsonic flight. Also the full-size is designed by very clever professionals with super computers, ours are designed by amateurs using rule-of-thumb approximations, so the full-size can do things quite differently because they have calculated exactly what is happening while we are guessing. And finally the full size is destabilised and as a result is fly by wire, whereas although our model is the same shape our model is most definitely stable with amuch more forward CG and does not have the luxury of fly-by-wire. So if we make a model of Spitfire which operates in the same subsonic region as us and has the same classic controls, we can make our model using the same rules. But when we model something that is supersonic and destabilised and fly-by-wire, but we still have to use the same Spitfire rules of subsonic, stable and classic control, we are going to see things that look bizarre to us and wonder if we should emulate the pivot point and balance of the full-size. H |
RE: balancing a full flying stab
1Eye, thanks for checking. I'm sure HarryC will forgive.
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RE: balancing a full flying stab
Guys!
I've been flying a BVM F-100 for Years (same one) no balance on the stab what so ever. It has never been an issue. |
RE: balancing a full flying stab
ORIGINAL: AndyAndrews 400 flights why bother? Why add extra weight to your jet if it doesn't really make any noticable difference, especially if the factories don't even recommend it in most cases? There's a huge difference between a factory not making any recommendation, and making a recommendation not to. I suspect most brands are simply not mentioning it? My AD F-100 instructions do mention it, it says to drill into the l/e of the stabs and put in lead to balance them both. H. |
RE: balancing a full flying stab
My AD F-100 instructions do mention it, it says to drill into the l/e of the stabs and put in lead to balance them both. This is the same process that was recommended by BVM in the manual they produced for Tam's F-18 when they were selling that model. BVM specifically recommended that the stabs should not be fully balanced on the F-18. I don't think Tam ever balances any of his flying stabs when he builds planes. I don't know the reasoning behind not fully balancing the stabs in models but this is why I have always let mine fall forward a little. I figured they know something I don't I guess... |
RE: balancing a full flying stab
ORIGINAL: Gary Jefferson they actually recommended to us is that you balance them to neutral and then remove 25% of the weight this is why I have always let mine fall forward a little. H |
RE: balancing a full flying stab
C-ARF say nothing about ballancing the all flying stabs on the Rookie or the Eurofighter, they are very light though and don't cause any buzz on the servo's in neutral, in flight the aerodynamic loading would diminish the stab loading when trimmed correctly with the C of G set at the optimum point, this all changes if you have the C of G a long way back for 3D flight when you would be carrying some down trim on the stab wich increase's the loading accordingly in straight and level flight, but how long does a airframe designed for 3D fly straight and level? LOL
Mike |
RE: balancing a full flying stab
..i was thinking if the elevon servo could be placed in the l.e. !??????
..this way we dont end up tail heavy each time by the extra waith we add to balance the stab...[&o] |
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